VORTICELLA 155 



temporary, while in the latter they are of different sizes, or het- 

 erogamous, and the fusion is permanent. As a result of this dif- 

 ferentiation in Vorticella one of the conjugating individuals is a 

 large, passive form, and the other is a small, active, motile form, 

 which finds and fuses with the passive form. A distinct fore- 

 shadowing of the two sexes which characterize the Metazoa is 

 thus present. 



Exercise 3. Look among a large number of Vorticellas for conju- 

 gating and for dividing individuals. Observe them carefully and 

 draw outlines of those observed. 



